The present invention relates to microelectromechanical (MEM) devices, and in particular to a dual-mass resonator structure which can be used, for example, to form microsensors that detect strain, acceleration, rotation or movement.
Microelectromechanical (MEM) devices are microminiature devices formed on a substrate using fabrication process steps common to the integrated circuit (IC) fabrication industry. These MEM devices generally combine electrical and mechanical functionality to form many different kinds of electromechanical devices including accelerometers, sensors, motors, switches, relays, coded locks, micromirrors and microfluidic devices.
Motion in the various kinds of MEM devices can be produced electrostatically through the use of comb actuators comprising a plurality of interdigitated fingers which produce relatively large displacements of up to 10 microns or more, but which require high operating voltages on the order of 100 volts or more. Alternately, parallel-plate electrostatic actuators without interdigitated fingers can be used to provide displacements of generally up to about 1 micron when operated at low voltages of about 20 volts or less. There is currently a need for MEM structures which are capable of providing relatively large displacements of several microns or more at low applied drive voltages of several volts for compatibility with integrated circuitry (e.g. CMOS circuitry). There is also a need for MEM resonator structures which are relatively insensitive to damping produced by an atmospheric ambient and to changes in pressure and temperature. Such MEM devices would be less costly to manufacture since they would not require vacuum encapsulation.
The present invention provides such a MEM structure which operates at low voltages (e.g. 4-15 volts) to provide displacements in a range of about 1-50 xcexcm, with the MEM structure being relatively insensitive to changes in pressure and temperature when operating near an antiresonant frequency.
The present invention relates to a microelectromechanical (MEM) structure, comprising a parallel-plate electrostatic actuator which further comprises a plurality of stationary electrodes formed on a substrate and a first mass suspended above the substrate to form a moveable electrode, with the first mass moving along a path in a plane parallel to the plane of the substrate in response to an actuation voltage applied between the stationary electrodes and the first mass; and a second mass suspended above the substrate and coupled to the first mass by a plurality of springs, with the second mass being driven by motion of the first mass to move along the same path. According to the present invention, each stationary electrode is located within a window formed through the first mass. Additionally, the second mass can optionally be located within a window formed through the first mass so that the first mass surrounds the second mass. The opposite arrangement is also possible with the first mass being located within a window formed in the second mass so that the first mass is surrounded by the second mass. In some cases, the second mass can be located adjacent to the first mass (e.g. when a pair of first masses are each arranged to drive a second masses, with the second masses being driven out-of-phase with respect to each other).
Various oscillatory modes of the MEM structure of the present invention are possible, with the second mass being driven to move along the same path as the first mass and either in-phase (i.e. in the same direction) with respect to the first mass or out-of-phase (e.g. 90xc2x0 or 180xc2x0 out-of-phase) with respect to the first mass. Oscillatory motion of the first mass can be further used to drive the second mass at or near a resonant frequency, or alternately at or near an antiresonant frequency. Finally, a contacting mode of operation is possible for the MEM structure 10. Such oscillatory motion as required for the various modes as described above can be produced using a cyclic (e.g. sinusoidal) actuation voltage.
An advantage of the present invention is that an extent of motion of the second mass can be larger than the extent of motion of the first mass upon actuation of the first mass. This is useful for providing motion of the second mass over a displacement of several microns or more while using a low-voltage (e.g. xe2x89xa615 volts) parallel-plate actuator which itself moves over a much more limited displacement range generally on the order of one micron or less.
Embodiments of the present invention can be formed as MEM structures providing either linear motion (i.e. along a straight path) or curvilinear motion (i.e. along a curved path). The MEM structures can further include a position-sensing electrode superposed with the second mass, and a ground plane underlying the first mass. Various embodiments of the present invention can be formed by surface micromachining utilizing a silicon substrate and one or more deposited and patterned layers of polycrystalline silicon for forming the first and second masses.
The present invention further relates to a MEM structure, comprising a substrate; a first mass suspended above the substrate and having a plurality of windows formed therethrough; a plurality of stationary electrodes formed on the substrate with each stationary electrode being located within one of the windows in the first mass, the stationary electrodes acting in combination with the first mass to form a parallel-plate electrostatic actuator to electrostatically move the first mass relative to the substrate along a path in response to an actuation voltage provided between the stationary electrodes and the first mass; a second mass coupled to the first mass through a plurality of springs, with the second mass being driven to move along the same path in response to motion of the first mass; and a position-sensing electrode located on the substrate below the second mass. The MEM structure can further include a ground plane formed on the substrate underneath the first mass.
The motion of the first and second masses can be oscillatory (e.g. using a sinusoidal actuation voltage, V=V0sinxcfx89t), with the extent of motion of the second mass being larger than the extend of motion of the first mass, and with the second mass, in some instances, moving along the path in a direction opposite that of the first mass. The second mass can be driven near a a resonant or antiresonant frequency by the oscillatory motion of the first mass, or can be driven in a contacting mode wherein the first mass is driven to contact at least a portion of the stationary electrodes. In embodiments of the present invention, the first mass can be fabricated to surround the second mass or to be surrounded by the second mass. The first and second masses can also be located proximate to each other, but not surrounding each other. Each of the first and second masses can be formed from one or more deposited and patterned layers of polycrystalline silicon (i.e. polysilicon). The MEM structure of the present invention can be operated at an actuation voltage that is generally xe2x89xa615 volts, and in some instances xe2x89xa65 volts.
The present invention also relates to a MEM structure, comprising a parallel-plate electrostatic actuator formed on a substrate and comprising a first mass suspended above the substrate and having a plurality of windows formed therethrough, and a plurality of stationary electrodes formed on the substrate at the locations of the windows in the first mass, the stationary electrodes acting in combination with the first mass to electrostatically move the first mass along a path in response to an actuation voltage provided between the stationary electrodes and the first mass; and a second mass located within one of the windows in the first mass, with the second mass being coupled to the first mass through a plurality of springs and further being driven to move along the same path as the first mass in response to motion of the first mass. Each spring coupling the first mass to the second mass can comprise a folded spring (i.e. a flexure).
The substrate can comprise, for example, silicon; and the first and second masses can comprise, for example, polysilicon. The second mass generally has a range of motion that is substantially larger than the range of motion of the first mass.
Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.